For many years a treatment, often one of last resort, for serious back problems has been spinal fusion surgery. Disc surgery, for example, typically requires removal of a portion or all of an intervertebral disc, which must then be replaced by a stabilizing structural element, such as a bony graft implant, that can maintain the separation between the vertebrae being fused so as to obviate damage to the spinal cord and adjacent nerves.
The most common sites for such surgery, namely those locations where body weight exerts its greatest loads on the spine, are in the lumbar region and immediately below, namely the vertebrae L1-2, L2-3, L3-4, L4-5, and L5-S1. However, other sites, for example the cervical vertebrae, are susceptible to related problems which also may require surgery and implants. In addition, a number of degenerative diseases and other conditions such as scoliosis require correction of the relative orientation of vertebrae, wherever they are located, by surgery and fusion.
In general for spinal fusions a significant portion of the intervertebral disk is removed and a stabilizing element, frequently including bone graft material, is inserted into the intervertebral space. It has been found in recent years that metal cages do not reliably produce fusion because they shield inserted bone growth material from the compressive loading that has been found to be necessary to promote new bone growth. Accordingly, other types of implants, especially those fabricated from autograft and allograft bone, have come into use. An example is the allograft implant of Nicholson, et al., U.S. Pat. No. 6,096,080, issued Aug. 1, 2000, which is an interlocking dovetail shaped device. Other forms of allograft and autograft implants are simply rectangular, cylindrical, or disc shaped and require no particular preparation of the adjacent vertebral end plates. Such bone grafts shapes are sometimes contoured to ride on the end plates of the vertebral bodies.
It has been found, however, that preparation of the end plates materially improves the fusion rate. Typically the surgeon prepares the vertebrae to be fused by breaking through, or cutting into, the hardened end plate surfaces of vertebral bone so as to allow an interposed bone graft or implant to come into direct contact with the cancellous (spongy) portion of the vertebral bone. Practitioners believe that this procedure produces superior results, first because it enables more intimate surface mating of the implant with the existing bone. Second, and more important, however, the cancellous bone interior to the vertebrae is vascularized, so that in the right circumstances blood will flow through the implant. Practitioners also believe that the clotting of blood inside such an implant is the first step of new bone growth. It is thought, therefore, that controlled “fracturing” of the vertebral endplates allows new bone generation to take place and for the incorporation of inserted bone grafts or implants into the two respective adjacent vertebral surfaces such that they become one continuous and rigid segment of bone.
In some cases, such as the dovetail implant, specialized cutting devices shaped specifically for that device will be provided. However, for more general shapes, such as discs and rectangular implants, it is not always possible to provide specialized instrumentation.
It is an object of the current invention to provide a method and an apparatus which can provide rapid and efficient preparation of an intervertebral space to receive an implant of a wide variety of shapes and configurations. It is another object of the present invention is to provide a device that can remove a sufficient amount of hard cortical bone from the respective endplates of the vertebral bodies so that blood from within the bones' medullar region can flow into and commence clotting within the region where the implant is to be installed. A further object of the present invention is to prepare the implant site in a way that does not destroy the structural integrity of or remove the endplate, but rather only abrades away as sufficient amount of the cortical material so that blood from the medullar regions can move into the region of the implant. Yet another more specific object of the invention is to provide a serrated cutting device that can be inserted into an intervertebral space in such a way as to remove controlled amounts of cortical bone simultaneously from the respective endplates of the two vertebral bodies being fused. And yet another objective of the present invention is to provide a method by which to keep the serrated cutting device from penetrating too deep into the intervertebral space. And a final objective of the present invention is to provide a method and a device for driving the serrated cutter into the intervertebral space with the least trauma to the spine.